<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .circle {
            display: block;
            position: absolute;
            border-radius: 200px;
            border: 1px solid #fc0;
            background: #fc0;
            width: 200px;
            height: 200px;

        }

        .start {
            top: -100px;
            left: -100px;
        }

        .end {
            bottom: -100px;
            right: -100px;
        }
        .container{
            border:1px solid black;
        }
        .main {
            width: 400px;
            height: 200px;
            /* 自适应auto */
            position: relative;
            background: #ccc;
            overflow: hidden;
        }

        .main::before {
            display: block;
            position: absolute;
            border-radius: 200px;
            border: 1px solid #fc0;
            background: #fc0;
            width: 200px;
            height: 200px;
            content: " ";
            top: -100px;
            left: -100px;

        }

        .main::after {
            display: block;
            position: absolute;
            border-radius: 200px;
            border: 1px solid #fc0;
            background: #fc0;
            width: 200px;
            height: 200px;
            bottom: -100px;
            right: -100px;
            content: " "
        }


        /*******************************/

        .container {
            line-height: 400px;
            height: 400px;
        }

        .center {
            /*水平居中*/
            text-align: center;
        }

        .main {
            /*垂直居中*/
            display: inline-block;
            vertical-align: middle;
            /* reset */
            text-align: left;
        }
    </style>
</head>

<body>
    <div class="container center">
        <div class="main">

            <!-- <div class="circle start"></div> -->
            adasdasdasasddasdasdasasfdasssssssssssssssssssssasdasdasdasd sdas
            <!-- <div class="circle end"></div> -->
        </div>
    </div>

</body>

</html>